The present disclosure relates to an image reading device that reads a document by shining light of a plurality of colors on the document, an image forming apparatus provided with such an image reading device, and a method of controlling an image reading device.
An image forming apparatus such as a multifunctional product/printer/peripheral or a copier includes an image reading device for copying, scanning, and other purposes. An image reading device includes, among others, a light source from which light is shone on a document, and an image sensor to which the light reflected from the document is shone back for conversion into an electrical signal. Some image reading devices read in colors. Some of them achieve color reading by turning on light sources of a plurality of colors (for example, three colors, like R (red), G (green), and B (blue)) cyclically, reading the different colors with an image sensor, and blending image data of the different colors into color image data.
One known image reading device is configured, specifically, as follows. By a lighting means, light sources of a plurality of colors (R (red), G (green), and B (blue)) are lit cyclically within each line to irradiate a document on a document stand, and by a focusing means, the resulting reflected or transmitted light is focused on a linear image sensor, so that a color image on the document is read. The image reading device further includes a transparent flat glass plate that is rotatable about the optical axis of the focusing means, a driving means for making the transparent flat glass plate rotate, and a controlling means for controlling the lighting means and the flat glass plate driving means. This configuration is aimed at eliminating a color displacement, which can be as large as ⅔ lines at the maximum, that occurs when data of three colors, R (red), G (green), and B (blue), are blended.
Conventionally, in an image reading device including light sources of a plurality of colors, within the width of one line determined from the reading resolution, the light source that is lit to emit light is switched. During the period for one line, reading for the plurality of colors is performed by an image sensor. Subsequently, based on the output of the image sensor, a plurality of colors' worth image data per line is generated. For example, in a case where three colors like R (red), G (green), and B (blue) are involved, during the period for reading one line, a red lamp, a green lamp, and a blue lamp are lit. As these lamps of different colors are lit, R (red), G (green), and B (blue) line data is generated as image data.
Here, reading and transferring the electric charges stored in the individual light receiving elements (photoelectric conversion elements) included in the image sensor requires a certain amount of time. Accordingly, compared with reading in black and white with the light sources of all the colors lit simultaneously, reading in three colors of R (red), G (green), and B (blue) requires three times the time (at one-third of the linear scanning speed). In other words, in reading in colors of R (red), G (green), and B (blue), reading is done three times per line, and thus reading one line takes, in color reading, three times the time required in black-and-white (single-color) reading. Thus, a color image cannot be read fast, and the reading speed in black and white is inferior to that in colors. In this way, color reading is disadvantageous in terms of productivity.
In the well-known image reading device mentioned above, an attempt is made to eliminate a color displacement that occurs when the color of the light source lit is switched a plurality of times within one line. However, compared with black-and-white (single-color) reading, color reading requires reading to be performed three time per line. This leads to poor productivity, and makes it impossible to read a color image fast. Thus, the image reading device mentioned above cannot cope with the disadvantage in terms of productivity. Furthermore, while the displacement to be eliminated is minute (for example, one dot or less), to eliminate it, the transparent flat grass plate needs to be positioned and operated accurately, and to that end, the transparent flat grass plate needs to be fitted accurately, and its rotation angle needs to be controlled accurately. Doing so, however, is rather unfeasible; in addition, securing high accuracy is also disadvantageous in terms of manufacturing cost.